LIQUEFACTION, FAULTING AND SHAKING EVIDENCE FROM MULTIPLE EARTHQUAKE EVENTS RECORDED IN THE LILY POINT CLIFFS AT POINT ROBERTS, WASHINGTON

Distinctive evidence of liquefaction, faulting and soft sediment deformation caused by multiple major earthquakes is displayed in the cliffs at Lily Point Washington. In this paper, we show evidence of clastic dikes, liquefaction squirts, sediment bubbles and flame structures exposed mainly in the lowest portions of the layered sandy cliffs. Frequent large landslides and coastal erosion expose the unit.

The most dramatic of the liquefaction features protrude upward and towards the NE (with a planar trend NW-SE dipping to the SW). These intrusions rise from an ancient buried channel filled with easily liquefied dark brownish gray silty material. The channel is clad with a rip-up deposit. This channel suggests a possible origin from a large flooding event not recorded again in the section despite several ice sheet overrunning events. The darker liquefaction prone material may have been deposited from backwash in a quieter flow environment after a big wave or flood. The most distinctive liquefaction features are noticed above the current sea level. Additional underlying deposits include “sand bubbles” where sand ellipsoids are encircled by formerly liquefied silt. The clay rich layers below the rip-up unit display soft sediment deformation.

Currently the sand cliffs are dry most of the time, and the material would not be expected to liquefy unless it becomes saturated. The liquefiable material is a good aquitard that preserved wood at base of the rip-up channel for more than 30 to 50 k years. Liquefaction squirts and flame structures likely occurred during the last (Sangamon) interglacial or when sea levels were 3 to 6 metres higher than present.

The large trees that remain at the top of the Lily Point Cliffs were damaged or topped by shaking events about 200 to 300 years ago, based on available tree ring studies . Minor shoreline displacements identified on Lidar images (from WA DNR) and pre Fraser glaciation topography also suggest a NW-SE trend of local faulting, folding and fractures.

The sand cliffs at Lily Point are not only very beautiful and accessible near the large Metro Vancouver region, but also share important geological stories to help improve decision-making related to the potential damage caused by large earthquake events.